1. Field of the Invention
The present invention relates to a circuit for a display, particularly to a driving circuit for an active matrix display, which includes amorphous-Si TFTs.
2. Description of the Prior Art
FIG. 1 is a diagram showing a conventional display driving circuit. The driving circuit 1 comprises a data electrode 11, a scan electrode 12 and a display cell composed of transistors 131, 132, capacitor 134 and light emitting diode 133. For clarity, there are only four display cells shown in FIG. 1. A data signal and scan signal are output from the data electrode 11 and scan electrode 12 respectively. In each of the display cells, the transistor 131 is turned on by the scan signal received by the gate, whereby the data signal is transferred to the gate of the transistor 132 through the transistor 131. There is a voltage difference generated by the data signal and stored on the capacitor 134. This voltage difference determines whether the transistor 132 is turned on. A current flows through and turns on the light emitting diode 133 when the transistor 132 is turned on; otherwise the light emitting diode 133 is turned off. Thus, by the scan signal sequentially scanning rows of the display cells, image data carried on the data signal is sent to each cell. Frames of image are formed by pixels with different luminance or colors which are determined by the light emitting diodes of the cells.
In the conventional display driving circuit described here, the transistor 131 may be an amorphous-Si TFT with low current driving ability since it is only used as a switch. On the contrary, the transistor 132 must be a poly-Si TFT with high current driving ability since it is used to drive the current flowing through the light emitting diode addition to being used as a switch. This results in a complicated manufacturing process, and increases the cost and decreases the yield of the display driving circuit manufacture.
However, it is impossible for all the transistors in the conventional display driving circuit to be amorphous-Si TFTs for the following reasons. First, the current driving ability of the amorphous-Si TFT is too low to provide a current high enough to turn on the light emitting diode. Second, the configuration in which the light emitting diode is disposed on the drain side of the transistor is disadvantageous to the manufacturing process of a driving circuit composed of the amorphous-Si TFTs.